A regal blue tang needs at least a 6' long tank. They grow really fast but more importantly they need swimming space regardless of size. They are open water swimmers and swim miles a day in the ocean. I have one the size of a nickle. She covers the entire length of my 6' tank. If they don't have enough space they get stressed and are more susceptible to disease and aggression. They are very susceptible to ich, as are all tangs. You should quarantine all new fish for at least 30 days to be sure they are disease free before you put them into your tank. Don't buy fish from *****. One fish with ich will infect the whole tank even if the fish are not showing symptoms it will be there. If you don't introduce it, it is not going to be an issue.
As for small colorful fish. You can't have Dori but you can have Nemo. Stick with an oscellaris or percula clown. Both are similar in appearance and stay relatively small and are pretty peaceful. You can get a watchman goby. I'd stay away from sand sifting gobies your tank is too small and too new. Royal gramma, orchid dottie back, clown goby, a cherub pygmy angel (other dwarfs need more space) a single blue damsel added last (stay away from domino, humbug, 3 or 4 stripe damsels too mean for small tank) they are all aggressive but truly beautiful. There are tons more I just can't think of them. A lot of online retailers have nano fish catagories which would be good to look through.
You can get a number of different invertebrates. Shrimp, hermit crabs, snails, a brittle star. I'd stay away from star fish or urchins for a while until your tank is stable, 6-12 months. No anemones until you tank is a year or more old and you have to have very strong lighting. If you want to do coral at some point light will be important.
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